AP-Micro Microbiology: Antimicrobial Susceptibility Testing 3

Question 1

Which test is used to detect inducible clindamycin resistance caused by the erm gene in staphylococci?

Accepted Answer

D-zone test (double-disk diffusion)

Answer

The D-zone test places erythromycin and clindamycin disks adjacent to each other; flattening of the clindamycin zone toward the erythromycin disk indicates inducible resistance.

Question 2

What does the term 'bactericidal' mean in the context of antimicrobial testing?

Accepted Answer

The drug kills at least 99.9% of the original bacterial inoculum

Answer

A bactericidal drug kills at least 99.9% (3 log reduction) of the original inoculum, distinguishing it from bacteriostatic agents that only inhibit growth.

Question 3

An organism that tests susceptible to a beta-lactam in vitro but carries a mecA gene should be reported as:

Accepted Answer

Resistant, because mecA confers oxacillin/methicillin resistance regardless of phenotypic results

Answer

mecA encodes an altered penicillin-binding protein (PBP2a) conferring MRSA resistance; organisms carrying mecA must be reported resistant to all beta-lactams regardless of in vitro appearance.

Question 4

What is the significance of the minimum bactericidal concentration (MBC) relative to the MIC?

Accepted Answer

MBC is the lowest concentration that kills 99.9% of bacteria, typically equal to or greater than the MIC

Answer

The MBC is the lowest drug concentration that kills 99.9% of the original inoculum and is equal to or higher than the MIC because killing requires higher concentrations than inhibition.

Question 5

Which method of AST is best suited for testing slow-growing or fastidious organisms like Haemophilus influenzae?

Accepted Answer

Disk diffusion on Haemophilus Test Medium (HTM) with appropriate supplements

Answer

Haemophilus Test Medium (HTM) provides the growth factors (NAD and hemin) needed for H. influenzae and gives reliable, reproducible susceptibility results.

AP Practice Test

AP Practice Test

AP-Micro Microbiology: Antimicrobial Susceptibility Testing 3